The present invention is related to guided missiles, fin general, and more specifically, to a guided missile subsystem including a Kalmanized radar track loop driven by acceleration signals generated by an inertial measuring unit, and a missile control loop driven by estimates of the relative kinematics of the missile and target computed by the radar track loop.
Missiles which are launched from aircraft in air-to-surface and air-to-air weapon delivery scenarios often include a guidance subsystem which operates to guide the missile on a collision course to a target. In some cases, the missile is launched after having acquired a target, wherein a search radar onboard the launch aircraft may initialize the guidance subsystem of the missile with the identified target cooridinates. In other cases, the missile may be launched prior to identifying a particular target, wherein the launch aircraft's radar may initialize the missile guidance subsystem to a patch on the ground or a target location estimated a priori. In this case, the missile guidance subsystem may identify and close in on a target within the specified ground patch or a priori target location.
In one embodiment, the missile guidance subsystem may include a self-contained radar on board the missile and may operate autonomously in an active mode. In other embodiments, the missile guidance subsystem may include only a radar seeker which operates in a semiactive mode; that is, the radar on the launch aircraft tracks and illuminates a particular target while the seeker in the missile picks up the back scatter from the launch aircraft radar, locks on and tracks the target back scatter until collision. In either embodiment, there exists no data link between the missile and the launch aircraft. Independent of whether the missile is operated in an active or semiactive mode, the guidance subsystem generally includes a radar processor and associated track loop or loops which provide feedback techniques to improve the accuracy of the missile guidance.
Generally, the radar tracking function is implemented with three partitioned track loops--a range track loop, a simple clutter or range rate track loop, and an angle track loop. The angle track loop may operate in conjunction with the radar processor to maintain the radar antenna boresight on identified target location. The range and range rate track loops may include integrators to provide an estimated range measurement and estimated range rate measurement for radar processing, respectively. In turn, the radar processor may compute the difference between corresponding actual and estimated range and range rate measurement and drive the corresponding integrator of the track loops directly with the appropriate computed difference. Because the tracking function does not fully take into account all of the real world kinematics of the missile motion, the model provided by the track loops may not be kinematically or dynamically exact. Accordingly, the range and range rate estimations generated thereby may produce errors in the missile guidance, commonly referred to as "dynamic lag errors".
The dynamic lag errors of the tracking loops may become quite large and troublesome especially when there exists a relative acceleration between the missile and the target. Generally, the way of coping with these large dynamic lag errors is to override them by cranking up the data rate measurement production of the radar processor. The data rate production of the radar processor may also have to be increased because of the inability of the conventional track loops to distinguish between angular and translational motion of the missile. To accomodate the missile guidance model imposed by the conventional track loops, the associated radar processor may be required to produce measurements at a relatively high data rate, say on the order of 30 Hz, for example.
These high data rate measurement constraints on the radar processor mey render insufficient time for processing the raw data received from the radar front end. Accuracy of the data measurements may be degraded because of the lack of time for adequate noise and clutter rejection. Moreover, with regard to synthetic aperture radar processors, insufficient processing time may result in incomplete motion compensation and nulling which results in a ground image generation of poor resolution. This poor image resolution may lead to greater miss distances of the missile with the target. Thus, it is of paramount importance to improve the modeling of the missile guidance dynamics in the tracking loops of the missile guidance system to permit a lowering of the measurement data rate of the associated radar processor and effect an improvement in radar measurement accuracy.
Another drawback of a conventional missile guidance subsystem is its sensitivity to the phenomenon known as angle glint, especially at short ranges, wherein the radar track loop which is fixed in frequency bandwidth becomes progressively less stable as the missile approaches the target. Consequently, the adverse effect of angle glint on the missile guidance is inversely proportional to the range of the target. Since angle glint is one of the chief contributors to target miss distances, ameliorating the phenomenon will lead to a reduction of the target miss distances.